1. Field of the Invention
The present invention relates to an image recording device, and more specifically to a temperature detection circuit that detects the temperature of a recording head performing image recording using thermal energy.
2. Description of the Related Art
Recording devices that record information such as desired characters and images, on sheet-shaped recording media such as paper and films, are widely used as information output devices for word processors, personal computers, facsimiles, etc.
As recording systems for recording devices, a variety of systems are known. Among them, the ink jet system has received special attention in recent years because it allows non-contact recording on a recording medium such as paper, facilitates colorization, and is superior in quietness. Out of the ink jet system, the serial recording system has been in general use because of its inexpensiveness, easiness in miniaturization, and the like. The serial recording system includes a recording head that discharges ink in accordance with desired recording information, and wherein recording is performed while conducting a reciprocating scan in a direction intersecting the feed direction of a recording medium such as paper.
In an ink jet recording device, an image is formed by discharging ink from a plurality of nozzles (discharge orifices) provided in a recording head onto a recording medium. In order to maintain satisfactory quality of the image formed in this manner, it is important to maintain constant size of ink drops, or the discharge amount of ink.
In an ink jet recording device that discharges ink drops using thermal energy, the size of ink drops generally depends on energy supplied for the discharge of ink drops, and the temperature of the recording head. The size of ink drops increases with the increase in the supplied energy or the head temperature. Therefore, in order to maintain constant size of ink drops, it is necessary to detect the temperature of the recording head, and control the energy to be supplied in accordance with the detected temperature.
Such temperature detection methods for a recording head are disclosed in patent documents, such as U.S. Pat. Nos. 5,638,097; 5,485,182; and 5,760,797; and Japanese Patent Laid-Open No. 5-050590.
As set forth in these patent documents, examples of known inexpensive temperature detection methods for a recording head include a method in which the temperature characteristic of the voltage drop (i.e., forward voltage drop in a diode) in a p-n junction of a semiconductor is used.
FIG. 7 is a perspective view showing a constructional example of a recording head. Reference numeral 100 denotes a heater board (element substrate), which is constructed by forming an electrothermal transducer (discharge heater) 105 and wiring 106 made of Al or the like for supplying power to the electrothermal transducer 105, on a silicon substrate by a deposition technique. A top plate 130 having partition walls for forming liquid passages (nozzles) 125 for a recording liquid such as ink, is adhered to the heater board 100, and thereby a recording head is formed.
Ink is supplied to a common liquid chamber 123 through a supply port 124 provided in the top plate 130, and is introduced from the common liquid chamber 123 into each nozzle 125. When the heater 105 is heated by energization, ink filled in the nozzles 125 generates bubbles, and ink drops are discharged from discharge orifices 126.
FIG. 8 is a plan view of the heater board 100 shown in FIG. 7. Reference numeral 5 denotes a discharge heater, and 4 denotes terminals to be connected to the outside by wire bonding. Reference numeral 2 denotes temperature sensors for temperature detection, each of which comprises a diode cell formed having the same size as that of a diode cell serving as a function element to be described later (in FIG. 8, A=B). Reference numeral 8 denotes a diode cell group for driving in which each diode cell comprises a diode function element having the same size as that of temperature sensor 2. Use of this diode cell group allows the discharge heater 5 to be selectively driven in accordance with image data.
Because the temperature sensors 2 are each formed as a function element comprising diodes or the like by semiconductor deposition processes as in the case of other portions, they can have a very high degree of accuracy. Furthermore, providing the temperature sensors 2 at both ends of the heater board (as shown in FIG. 8) allows the distribution state of the substrate temperature in the array direction of the nozzles to be grasped from outputs of the temperature sensors.
FIG. 9A is a diagram showing a construction wherein a diode 20 comprising a p-n junction is formed on the heater board 100, and wherein the diode 20 is used as a temperature sensor 2 by using its diode characteristic. As can be seen from FIG. 9A, Al electrode wiring 18 is led out from each of the p-region and n-region of the diode 20, and an insulating layer 19 of SiO2 is formed between the substrate surface and the diode 20.
FIG. 9B shows an equivalent circuit of the diode shown in FIG. 9A. In FIG. 9B, when a current flows from “A” towards “B”, a forward voltage drop VF in the diode occurs. In general, the forward voltage drop VF varies in accordance with temperature variation, and the detection of temperature is performed by making use of this variation.
FIG. 1 shows an example of a conventional temperature detection circuit using a diode sensor. The pre-stage circuit is a constant-current circuit for feeding a constant current through the diode sensor. Specifically, the constant current determined by a voltage Vref divided by resistors 10 and 11, and the resistance value R1 of the resistor 12, that is, Vref/R1 flows through the diode sensor 14 irrespective of the environmental temperature during temperature detection, this current value being, for example, on the order of 200 μA.
The voltage drop VF in the diode sensor 14 relative to the point “a” in FIG. 1 decreases with the increase in temperature, as indicated by a straight line 22. The temperature gradient at this time is, for example, −2.1 mV/° C. Therefore, provided that the temperature of the recording head increases by 55° C., for example, from 25° C., which is room temperature, to 80° C., which is a detected temperature indicating an excessive temperature rise of the head, the voltage drop VF decreases by 115 mV (that is, ΔVF=−115 mV).
On the other hand, the post-stage circuit shown in FIG. 1 is a voltage amplification circuit, which amplifies the variation ΔVF of the voltage drop VF in the diode sensor 14, detected in the pre-stage constant current circuit in accordance with a gain determined by the ratio of the resistance value R3 of the resistor 16 to the resistance value R2 of the resistor 15, that is, R3/R2. In this manner, ΔVF is amplified as shown by the straight line 24 in FIG. 2 so that ΔVF fits to the dynamic range of the analog/digital converter (A/D converter).
Here, if we suppose that the dynamic range of the A/D converter is 2.5 V, and that its resolution is 8 bits, i.e., 256 steps, then 1 step corresponds to 9.77 mV. Therefore, the absolute value (=115 mV) of the variation ΔVF (=−115 mV) of VF corresponds to 12 steps. This results in utilizing only a range corresponding to about 5% of the dynamic range 2.5 V. In this way, the conventional temperature detection circuit is difficult to make an effective use of the dynamic range of the A/D converter. This makes the conventional temperature detection circuit susceptible to noise. Also, in the construction shown in FIG. 1, with no noise provided, the inputs V+ and V− of the operational amplifier 13 are equal to each other, and maintain a state of equilibrium. However, when positive noise due to common mode noise or the like intrudes on the wiring connected to the diode sensor 14, the absolute value of V− may become higher than that of V−, thereby producing a loss of equilibrium. In such a case, the potential at the point “b” in FIG. 1 enters a low level, and herein, undesirably becomes 0 V. Conversely, when negative noise intrudes on the wiring, the potential at the point “b” in FIG. 1 enters a high level and undesirably becomes Vcc. In this manner, the construction as shown in FIG. 1 is susceptible to noise connected to the diode sensor 14, thereby causing a possibility of erroneously detecting an excessive temperature rise.
Furthermore, as is well known, the A/D converter generally maintains the linearity in the vicinity of the center of the dynamic range as shown in FIG. 3, but tends to lose the linearity in the regions near the power supply voltages, i.e., 0 V and Vcc.
FIG. 3 is a graph showing, by comparison, an example of the characteristic of an ideal A/D converter and that of the characteristic of a low-cost A/D converter, which cannot maintain the above-described linearity. The dotted line 30 in FIG. 3 represents an example of the input/output characteristic of the ideal A/D converter, while the solid line 32 represents that of the characteristic of a low-cost A/D converter. As illustrated in FIG. 3, the low-cost A/D converter exhibits linearity in the region indicated by “C”, near the center of the dynamic range, but it loses the linearity in the “A” and “B” regions in the vicinity of both ends of the dynamic range, i.e., near the power supply voltages (0 V and Vcc). Although an A/D converter improved in such a tendency is available, it is generally expensive.